Effective tumor treatment is frequently thwarted by the lack of sensitivity of certain tumors to standard chemotherapeutic agents (intrinsic resistance) or by the ability of certain tumors to develop a lack of chemotherapeutic sensitivity during the course of treatment (acquired or extrinsic resistance). The cause of these phenomena has been linked to the existence of an energy dependent efflux pump which acts to remove the chemotherapeutic agent from the target cell. The pump consists of the P-glycoprotein found as a constituent of cell membrane, and it has been suggested that the normal function of the P-glycoprotein is to remove toxins from within the cell. This theory is supported by the observation that P-glycoprotein is found as a cell membrane constituent in cells of liver, kidney, colon, and jejunum tissues. It has been suggested that P-glycoprotein in the cell membrane of such normal tissues could act to remove toxins or to assist in the transport of nutrients and solutes and to secrete a variety of protein and steroid substances. The natural presence of P-glycoprotein in tumor cells derived from these tissues as well as its presence in tumor cells derived from other tissue types could explain, at least in part, resistance of various tumors to therapy with standard chemotherapeutic agents. The use of agents which inactivate the P-glycoprotein pump could be therapeutic and valuable in the treatment of multi-drug resistant tumors.